Controlled preparation and growth of functional nanomaterials, including semiconductor quantum dots, carbon nanotubes, and metal oxides, have received considerable attention in the literature and industry because of the potential impact on lucrative areas of electronics, energy production and storage, medicine, and chemical catalysis. Specifically, silica nanoparticles, e.g., SiO2 NP and variants thereof, offer ideal properties, such as transparency to visible wavelengths, dielectric properties, high surface area, ease of functionalization, and relatively low toxicity. These physical properties make silica an attractive material for embedding or encapsulating other materials to form a functionalized protective shell.
Conventional methods to synthesize SiO2 nanomaterials include the Stöber method and the water in oil (w/o) reverse microemulsion method. Both methods involve hydrolysis and condensation reactions of a siloxane source, such as tetraethyl orthosilicate, catalyzed by mineral acids, ammonia, alkali metal hydroxides, and fluoride containing salts. Methods catalyzed by alkaline conditions tend to form sols; acid-catalyzed methods tend to form gels. Variants of both methods include changes in concentration, time, temperature, pH, surfactant, and the use of an additional catalyst. Specific examples are provided in Equations 1-3.
                    ≡                  Si          -                      O            ⁢            R                    +                                    H              2                        ⁢                                                            O                  ⇌                  ≡                  S                                esterification                            hydrolysis                        ⁢            i                    -          OH          +                      R            ⁢            OH                                              Equation        ⁢                                  ⁢        1                                ≡                  Si          -                      O            ⁢            R                    +          HO          -                                                    Si                ≡                ⇌                ≡                Si                                                              alc                  ⁢                  ohol                                ⁢                                                                  ⁢                condensation                                      alcoholysis                    -          O          -          Si                ≡                  +                      R            ⁢            OH                                              Equation        ⁢                                  ⁢        2                                ≡                  Si          -          OH          +          HO          -                      Si            ⁢                                                            ≡                  ⇌                  ≡                                hydrolysis                                            water                ⁢                                                                  ⁢                condensation                                      ⁢            Si                    -          O          -          Si                ≡                              +                          H              2                                ⁢          O                                    Equation        ⁢                                  ⁢        3            
Several parameters may affect the equilibrium of these reactions, including, for example, choice of silicon alkoxide precursor, nature of the catalyst, concentration of silicon alkoxide, [H2O]/[siloxane precursor] ratio, choice of solvent, temperature, and pressure. Additionally, a length of the alkoxide group of the siloxane precursor may directly affect the rate of hydrolysis, for example, wherein methoxy- reacts more quickly than ethoxy-, which reacts more quickly than butoxy-. Alkaline catalyzed reactions are commonly favored for SiO2 NP synthesis—the increased reactivity under alkaline catalysis reactions result in immediate condensation upon hydrolysis, whereas stable monomers can be formed using acidic conditions.
SiO2 NPs have also been synthesized by irradiating Stöber reaction solutions at 2.45 GHz using both laboratory microwave reactors and kitchen microwave ovens, although reported results claim uncontrolled growth and a high degree of polydispersity of the silica product. The popularity of microwave-assisted chemistry is not surprising considering these methods often dramatically increase yields, decrease reaction times, and, many times, allows for solvent-free reactions. In microwave chemistry, all components of the reaction (e.g., reagents, solvents, and vessels) are capable of interacting with, or otherwise perturbing, the electromagnetic (“EM”) field. Molecular species with permanent dipoles align with the electric field and, through molecular rotation, generate thermal energy (heat) via molecular friction. Dielectric properties of non-conductive material govern the manner in which the material heats when exposed EM fields. A loss factor, tan δ, is a measure of the ability for a material to convert EM energy into heat at a given frequency and temperature. Solvents may be categorized by a loss factor, wherein high tan δ solvents have values greater than about 0.5, medium tan δ solvents have values ranging from about 0.1 to about 0.5, and low tan δ solvents have values less than about 0.1. High loss factor solvents (for example, ethanol, 2-propanol, and methanol having tan δ values of 0.941, 0.799, and 0.659, respectively) have been used in the preparation of SiO2 NPs because a polar solvent is required for solubility of the siloxane precursor.
Despite these improvements in SiO2 NP synthesis, there remains a need for still further improved methods for synthesizing SiO2 NPs, such as methods that result in increased yields, short reaction times, and precise control of size and morphology.